1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a facsimile apparatus, or a printer, which has an endless belt wound and stretched between a plurality of rollers and adapted to rotate and which forms an image on a recording material by utilizing an electrophotographic system.
2. Related Background Art
There have conventionally been proposed various image forming apparatuses, which are equipped with a plurality of image forming portions employing an electrophotographic system, and in which toner images of different colors are respectively formed on photosensitive members of the image forming portions, the toner images being successively transferred to the same recording material and superimposed one upon the other to thereby form a color image. Further, there have been proposed image forming apparatuses, such as color copying machines which are equipped with an endless transferring belt adapted to bear and convey a recording material (or an intermediate transferring belt adapted to bear and convey toner images) in order to perform high-speed color recording, a plurality of image forming portions being linearly arranged along the transferring belt.
In such image forming apparatuses, the transferring belt is wound and stretched between a driving roller and a plurality of driven rollers (rocking rollers, tension rollers, etc.), and foreign matter adhering to the surface of the transferring belt, such as toner and paper dust, is removed by a cleaning means.
In the above-described conventional image forming apparatuses, there is a fear that scattered foreign matter, such as toner and carrier, is allowed to get to the inner side of the transferring belt and adheres to the surface of the rollers between which the belt is wound and stretched. This might be prevented by providing a new cleaning means for cleaning the reverse surface of the transferring belt.
However, while in such a construction the reverse surface of the transferring belt is cleaned, there is a fear that some foreign matter, which has not been removed by the above-mentioned cleaning means, will be allowed to gradually adhere to the surface of the driven rollers. Since the foreign matter adhering to the driven rollers is not easily separated to be transferred to the reverse surface of the belt, it is difficult to remove the adhered foreign matter from the driven rollers.
When foreign matter is thus allowed to adhere to the driven roller, the transferring belt coming into contact with the portion of the driven roller where the foreign matter exists will come to have a convex portion by the foreign matter. The cleaning means (e.g., cleaning blade) for cleaning the belt surface does not abut the convex portion of the transferring belt in a satisfactory manner, resulting in a cleaning defect. And, foreign matter remaining on the belt surface (stain) as a result of such cleaning defect will adhere to a reverse surface of the coming recording material P to stain the same. Further, the convex portion will make it difficult to attain uniform contact, resulting in a transfer defect.
This problem might be solved by forming the surface portions of the rollers between which the transferring belt is wound of an elastic material, such as rubber. When the roller surface consists of rubber, if foreign matter adheres to the roller and gets in between the roller and the transferring belt, the convex portion due to the foreign matter adhering to the roller is absorbed by the elasticity of the rubber, so that the transferring belt suffers no such damage as described above.
However, when the roller surface portion is formed of rubber, the frictional force between the roller and the transferring belt will increase. And, it has been found as another problem that this change in the frictional force makes the running of the transferring belt unstable.
This will be discussed in more detail. The rocking of the transferring belt (the lateral movement of the transferring belt in a direction perpendicular to the running direction of thereof) is determined by the advancing direction of the transferring belt and the roller angle. Thus, when the plurality of rollers exhibit a great frictional force, a force is applied which causes the respective rollers to rotate the transferring belt in different directions (the axial directions of the rollers), so that the running of the transferring belt becomes unstable. For example, the transferring belt is offset until it is detached from the rollers, with the result that an end portion of the belt is damaged. It is to be noted that the larger the frictional force of the driving roller, which transmits driving force, the better. Thus, the running of the transferring belt will be more stable if exclusively the surface of the driving roller consists of rubber.
In addition, when in such an apparatus toner adheres to the driven rollers to cause a change in frictional force, there is also a fear that the rocking control of the belt will become unstable.
The present invention has been made in view of the above problems. It is an object of the present invention to prevent generation of a local convex portion as a result of adhesion of foreign matter, such as scattered toner, to the rollers between which the transferring material carrying belt or the intermediate transferring belt is wound and stretched, thereby preventing staining of the rear surface of the transfer material due to cleaning defect of the belt, transfer defect, etc.
In order to achieve the above-mentioned object, according to a preferred embodiment of the present invention, there is provided an image forming apparatus including: an image forming means for forming an image; a transferring material conveying belt for conveying a transferring material toward the image forming means; a driving roller for driving the transferring material conveying belt; a driven roller around which the transferring material conveying belt is wound; and a transferring means for transferring the image formed by the image forming means to the transferring material, in which the surface of the driven roller has a layer whose coefficient of friction is lower than that of the surface of the driving roller.
According to another preferred embodiment of the present invention, there is provided an image forming apparatus including: an image forming means for forming an image; an intermediate transferring belt to which the image formed by the image forming means is transferred; a driving roller for driving the intermediate transferring belt; a driven roller around which the intermediate transferring belt is wound; and a transferring means for transferring the image on the intermediate transferring belt to a transferring material, in which the surface of the driven roller has a layer whose coefficient of friction is lower than that of the surface of the driving roller.